Vietnam's Corps of Engineers say the explosion that killed four children and injured four others in the Mekong Delta on December 2 came from a mortar shell of the type used by the US military and the South Vietnam regime during the Vietnam War.

Lieutenant Colonel Pham Hong Hung of the Vinh Long Military High Command said the UXO (unexploded ordnance) was a 61-mm mortar shell with an antipersonnel range of up to 30 meters in diameter.

The tragedy occurred around 1 p.m. near the home of Nguyen Van Lot in Vung Liem District's Hieu Nghia Commune, where locals had gathered for the death anniversary of Lot's mother.

The children were playing in the bamboo bush when the mortar shell exploded. It is still unclear what precisely prompted the explosion.